One of the major impediments to the commercialization of an amperometric sensor, particularly an enzymatic glucose sensor, is its settling time. Settling time is the amount of time necessary for the current output from the sensor to settle to a stable value following the initial application of the potential to the sensor. During this time, the electrical double layer at the surface of the working and counter electrodes is charged and faradaic reactions are established.
With amperometric, electroenzymatic glucose sensors, settling times can take up to several hours. For example, Koudelka et al. have reported a 90-minute settling time following in-vivo implantation. Koudelka, Rohner-Jeanrenaud, Terrettaz, Bobbioni, Rooij & Jeanrenaud, In-Vivo Behavior of Hypodermically Implanted Microfabricated Glucose Sensors, 6 Biosensors & Bioelectronics 31 (1991). Similarly, Velho et al. reported a settling time of at least one hour for an implanted needle-type glucose sensor. Velho, Sternberg, Thevenot & Reach, In Vitro and In Vivo Stability of Electrode Potentials in Needle-Type Glucose Sensors, 38 Diabetes 164 (1989). Between two and four hours of settling time were required for an in-vivo sensor fabricated by Rebrin et al. Rebrin, Fischer, Woedtke, Abel & Brunstein, Automated Feedback Control of Subcataneous Glucose Concentration in Diabetic Dogs, 32 Diabetologia 573 (1989). Similar settling times have been observed and reported by other investigators. It can be appreciated that these settling times are a disadvantage to the commercialization of an amperometric sensor, and that such settling times may preclude the use of electrochemical sensors in emergency care circumstances.
In one aspect of the present invention, the settling time of an electroenzymatic glucose sensor is reduced by pretreating the working electrode of the sensor. Although such a method has not previously been disclosed, electrochemical pretreatment of electrodes is known for other purposes. For example, electrochemical methods have been used to remove contaminants from the surface of noble metal electrodes, to enhance the sensitivity and selectivity of the electrodes, and to shift the oxidation potential of some compounds. However, the electrochemical pretreatment of an amperometric glucose sensor for the purpose of reducing its settling time is not suggested by such treatments, and has not been heretofore reported.
Another problem encountered with the use of electrochemical sensors to measure the concentration of a particular compound such as glucose is "interference" caused by the oxidation of other compounds in the body. For example, common interfering compounds encountered in in-vivo implantation include ascorbic acid, uric acid, cysteine and acetaminophen. These interfering compounds may cause an erroneous positive offset which is not acceptable when quantifying the glucose levels of individuals with diabetes.
Several methods have been devised to minimize the effects of interfering compounds on electroenzymatic glucose sensors. For example, a regenerated cellulose membrane or a negatively charged cellulose acetate membrane has been incorporated between the working electrode and the enzyme layer covering that electrode. These membranes allow the hydrogen peroxide to permeate the membrane layer, but interfering compounds such as ascorbic acid and uric acid cannot pass due to the size and/or charge exclusion characteristics of the membrane.
In another technique, two electrodes are used. One electrode is covered with an oxidizing enzyme, while the enzyme is denatured or absent from the other electrode. The current from interfering compounds is eliminated by analyzing the difference in current output from the two electrodes.
These techniques for reducing sensor sensitivity to interfering compounds are not attractive because additional membranes and/or electrodes increase the cost to produce the sensors. In addition, including an extra membrane layer to exclude interfering compounds would both increase the response time of the sensor and decrease the magnitude of the current output.
Still another technique incorporates a chemical that is capable of functioning as an electron acceptor in place of oxygen to shuttle electrons from the redox center of the enzyme to the surface of the working electrode. These chemicals are referred to as redox mediators. This type of sensor is reported to be oxygen-insensitive and can function in an anaerobic environment. These compounds have a low redox potential which lessens the chance of interferences from other electroactive compounds. Unfortunately, these compounds are difficult to retain inside the sensor and some are toxic.
A need therefore exists for a method of improving the utility of electrochemical glucose sensors by reducing their settling time and by improving their ability to provide accurate readings of one desired compound despite the presence of interfering compounds. The present invention addresses these needs.